The Catalytic Subunit of the System L1 Amino Acid Transporter (Slc7a5) Facilitates Nutrient Signalling in Mouse Skeletal Muscle

Abstract

The System L1-type amino acid transporter mediates transport of large neutral amino acids (LNAA) in many mammalian cell-types. LNAA such as leucine are required for full activation of the mTOR-S6K signalling pathway promoting protein synthesis and cell growth. The SLC7A5 (LAT1) catalytic subunit of high-affinity System L1 functions as a glycoprotein-associated heterodimer with the multifunctional protein SLC3A2 (CD98). We generated a floxed Slc7a5 mouse strain which, when crossed with mice expressing Cre driven by a global promoter, produced Slc7a5 heterozygous knockout (Slc7a5+/−) animals with no overt phenotype, although homozygous global knockout of Slc7a5 was embryonically lethal. Muscle-specific (MCK Cre-mediated) Slc7a5 knockout (MS-Slc7a5-KO) mice were used to study the role of intracellular LNAA delivery by the SLC7A5 transporter for mTOR-S6K pathway activation in skeletal muscle. Activation of muscle mTOR-S6K (Thr389 phosphorylation) in vivo by intraperitoneal leucine injection was blunted in homozygous MS-Slc7a5-KO mice relative to wild-type animals. Dietary intake and growth rate were similar for MS-Slc7a5-KO mice and wild-type littermates fed for 10 weeks (to age 120 days) with diets containing 10%, 20% or 30% of protein. In MS-Slc7a5-KO mice, Leu and Ile concentrations in gastrocnemius muscle were reduced by ∼40% as dietary protein content was reduced from 30 to 10%. These changes were associated with >50% decrease in S6K Thr389 phosphorylation in muscles from MS-Slc7a5-KO mice, indicating reduced mTOR-S6K pathway activation, despite no significant differences in lean tissue mass between groups on the same diet. MS-Slc7a5-KO mice on 30% protein diet exhibited mild insulin resistance (e.g. reduced glucose clearance, larger gonadal adipose depots) relative to control animals. Thus, SLC7A5 modulates LNAA-dependent muscle mTOR-S6K signalling in mice, although it appears non-essential (or is sufficiently compensated by e.g. SLC7A8 (LAT2)) for maintenance of normal muscle mass.

Specific targeting of TGF-β family ligands demonstrates distinct roles in the regulation of muscle mass in health and disease

“…Phosphorylation of effector molecules in the insulin-like growth factor (IGF)-stimulated protein synthesis pathway, mTOR and S6RP, was significantly enhanced after combined activin B and myostatin prodomain treatment. In addition, RNA-Seq showed that complete inhibition of Smad2/3 signaling in muscle resulted in markedly increased expression of growth factor (Igf2, increased 2.4-fold), amino acid transport (Slc7a5, fourfold; Lat2, 3.8-fold), and amino acid biosynthesis (Psat1, 3.7-fold) genes critical for protein synthesis (33)(34)(35). In terms of protein degradation, the expression of the E3 ubiquitin ligases Fbxo32 (MAFbx/atrogin1), Trim63 (MuRF1), and Fbxo30 (Musa1) (7,22), which classically drive this pathway in muscle, was not substantially reduced after 8 wk of myostatin/activin blockade.…”

Transport of Iodothyronines by Human L-Type Amino Acid Transporters

“…LAT4 is expressed in placenta, kidney, liver and small intestine (25,(31)(32)(33)(34)(35). Table 1 presents an overview of the tissue distribution of LAT1, LAT2, LAT3, LAT4 and LAT5 (26,28,29,(31)(32)(33)(35)(36)(37)(38)(39).…”

Amino Acid Transporters in Cancer and Their Relevance to “Glutamine Addiction”: Novel Targets for the Design of a New Class of Anticancer Drugs

“…Therefore, these transporters have potential as drug targets in cancer therapy. In contrast, deletion of Slc7a5 in mice is embryonically lethal (49). This transporter is essential for the transfer of amino acids from blood into brain across the blood-brain barrier, suggesting a critical role for the transporter in the function and development of the brain.…”